1. Field of the Invention
This invention relates to connector plates, and more particularly to metal connector plates used in prefabricating wooden structural members such as roof trusses.
2. Description of the Prior Art
Metal connector plates in various configurations have been utilized to join together wooden components for forming structural members such as roof trusses, box beams and pallets. Generally the connector plates are formed from flat metal plates in which slender, nail-like teeth are punched or struck out to extend generally perpendicularly outwardly from one face of the plate. A void or slot is left in the plate at the locations in which the teeth have been punched out.
To prefabricate wooden structures, such as roof trusses or box beams, wooden members are first arranged in coplanar relationship to each other and then connector plates are placed over the various joints so that the side of the connector plate from which the teeth are struck out lies adjacent the wood members. Usually, a connector plate is positioned on each side of the wood members. Then the teeth of both connector plates are simultaneously driven into the wood members by well-known means such as through the use of either a C-shaped hydraulic ram press or by a gantry type roller press. Once embedded, the teeth securely fasten the connector plates to the wooden members to enable the plates to keep the wooden members joined together while transmitting loads between the members of the wooden structure.
Wooden roof trusses constructed with connector plates of the type described above have been found to be stronger than conventional nailed roof trusses of equal weight. Thus, for a given load and span, trusses incorporating connector plates can be made from smaller sized lumber thereby decreasing construction costs. Moreover, the use of metal connector plates enables roof trusses to be prefabricated at a central manufacturing location at a cost significantly less than that required to construct roof trusses at particular job sites in the traditional fashion wherein each member of the truss is individually cut and nailed together by workmen.
Nevertheless, connector plates and wooden roof trusses constructed with connector plates are not without deficiencies. When truss joints are highly stressed, connector plates often either pull out of the wood or alternatively the plate itself fails. To forestall failure of the truss joints under load, the teeth of the connector plate have been formed in various configurations and lengths. Furthermore, in an attempt to improve the load carrying capacity of truss joints, teeth have been struck out from the plate material in a wide variety of patterns.
When wooden truss members are heavily loaded during use, conventional connector plates have a tendency to pull out of the wood. In an effort to prevent the pulling out of the teeth, conventional connector plates have been constructed with a large number of teeth per unit area of connector plate thereby increasing the surface area and number of teeth against which the wood fibers bear when the truss is loaded. Reducing the load that each tooth is required to carry correspondingly decreases the likelihood that the teeth will pull back out of the wood.
One manner of increasing the number of teeth per unit area of connector plate is by striking the teeth out in pairs thereby leaving only one slot in the plate material for each two teeth. As illustrated by Jureit et al, U.S. Pat. No. 3,892,160, striking the teeth out in pairs permits twice as many teeth to be struck out per unit of connector plate while still leaving a maximum amount of plate material surrounding each tooth thereby minimizing the reduction in the strength of the plate itself. Other examples of connector plates having teeth struck out in pairs are disclosed by Templin et al, U.S. Pat. Nos. 3,277,768; Sanford 3,498,170; Wood, 3,603,197; and, Moehlenpah, 3,951,033.
However, increasing the number of per unit area of connector plate may have negative effects. As the number of teeth is increased, the tensile strength of the plate itself is lowered due to the reduced amount of metal left in the plate after the teeth have been struck out. If too many teeth are struck out, the plate is weakened to the point that it is incapable of carrying the tensile loads imparted by the wood members thereby resulting in failure in the plate itself. Furthermore, as the number of teeth per unit area of connector plate is increased, the tendency of the wood fibers to split when the plate is pressed into the wood members increases so that the structural strength of the wooden members themselves is diminished. Thus, there is a limit to which wooden joints can be strengthened by simply increasing the number of connector plate teeth.
Other attempts to prevent connector plate failure, include constructing the teeth with one or more laterally extending barbs to thereby theoretically lock wood fibers between the surface of the connector plate and the barb. Examples of connector plates utilizing this type of tooth construction are disclosed by Menge U.S. Pat. No. Re. 28,427; Menge U.S. Pat. Nos. 3,011,226; Foley et al 3,090,088; Carr 3,266,362; and, Wood 3,603,197. However, as indicated, the extent to which wooden joints can be strengthened by utilizing barbed teeth is limited. If the barbs extend very far laterally from the primary portion or shank of the tooth, the adjacent wood fibers will be cut or severed as the tooth is pressed into the wood member. As a result, the load carrying capacity of the joint is actually decreased.
In another type of known connector plate, the teeth are configured to deform or bend as they are driven into the wooden members to thereby lock wood fibers between the tooth and the surface of the connector plate. In some such connector plates, the teeth are formed with asymmetrically shaped tip portions. The reaction of the asymmetrical tip portions against the wood fibers creates a force component acting perpendicularly to the height of the teeth thereby causing the teeth to deflect as they are driven into the wooden members. Examples of connector plates having this type of tooth construction are disclosed by Black et al, U.S. Pat. Nos. 3,382,752; Moehlenpah 3,417,651; and, Jureit et al 3,892,160.
In yet another type of connector plate utilizing teeth that deflect as they are pressed into the wood, the tip portions are nominally bent or askewed relative to the remainder of the tooth. The bent tip portion of the tooth acts as a camming surface which produces a reaction force acting in a direction across the height of the tooth causing the teeth to deform laterally as they are driven into the wood. Examples of connector plates incorporating this type of tooth configuration are disclosed by Foley et al. U.S. Pat. Nos. 3,090,088; Moehlenpah et al 3,322,018; Koenigshof 3,343,439; and Black et al 3,382,752.
In a further attempt to prevent connector plates from withdrawing from wooden members, the teeth have been twisted in the manner resembling the configuration of a corkscrew. Examples of this type of connector plate is disclosed by Schmitt U.S. Pat. Nos. 3,633,454; Jureit et al 3,892,160 and 4,031,803, wherein the teeth are twisted along their height. A connector plate with this particular tooth configuration also has been manufactured by P. H. Bowman Company, Inc. of Seattle, Wash.
Another common manner of connector plate failure occurs when the plates are loaded in a direction transversely of the face of the struck out teeth. In this direction of loading there is considerably less tooth area for the wood fibers to bear against so the teeth can withstand a smaller load then if they are loaded in a direction normal to their face. In an attempt to overcome this shortcoming, connector plates, as exemplified by Foley et al U.S. Pat. Nos. 3,090,088 and Black et al 3,382,752, have been constructed with sets of teeth disposed both longitudinally and transversely to the length of the connector plate. Also, connector plates have been constructed with generally flat or planar teeth which are askewed or rotated relative to the longitudinal axis of the connector plate to thereby increase the surface area of the tooth in the direction transverse to the length of the connector plate. An example of this type of connector plate is disclosed by Moehlenpah U.S. Pat. No. 3,951,033.
In spite of the various types of connector plate construction thusly described, applicant is not aware of any connector plate which optimumly incorporates all of the desirable characteristics of connector plates. Some of the known connector plates provide a large number of teeth in an attempt to prevent withdrawal of the connector plate from the wooden members but as a result compromise the strength of the connector plate itself causing it to fail when loaded. Other connector plates provide a sufficient capacity to carry loads which act longitudinally of the connector plate but are unable to withstand any substantial load in the direction transverse to the length of the connector plate. Furthermore, although some connector plates are designed to lock wood fibers between the teeth and the connector plates, they also gouge or tear the wood fibers thereby substantially decreasing the load carrying capacity of the wood members themselves.